Interim Report on Fuel Pin Bowing - Initial Method

A method is presented for examining the bowing phenomenon of fuel pins of symmetrical cross section located loosely in a moderator block channel or carrier which is assumed to provide straight and unyielding reaction surfaces. The method adopts the elementary approach of a model constructed in a two component material spliced about its central axis, using as a starting point the analysis of bi-metallic strips. In order to study the behaviour under time dependent effects arising from irradiation damage, it is assumed that, throughout its residence time in the reactor, the pin is subjected to a uniform temperature gradient at a fixed orientation. Shutdown effects are also examined when the thermal gradient bowing is removed temporarily. In a simple pin problem given as a detailed example of the method, the principal influences taken into account are dimensional change and creep of the graphite. The treatment may be considered as a crude forerunner of a multi-layer analysis, necessitating solutions by computer, which is under development. (author)